psu-ng: Fixed PSU monitor app to bind/unbind device driver appropriately.

Modified bindOrUnbind function:
      Bind the  device driver when the PSU present and i2cbus-i2caddr
      does not exist. Unbind the device driver when i2cbus-i2caddr exists
      and the PSU not present.

      bindOrUnbind does not do anything when:
      * PSU device driver bind exist and the device present
      * PSU device driver bind does not exist and device not present

Added new function populateDriverName:
      Search for device driver name in all PSUs, then assign the device
      driver name to all PSUs including missing PSUs.

Testing:
    Verified in the following test, there is no bind/unbind error log.
        1 - Removed 1 PSU and powered on the BMC with 3 PSUs
        2 - Use step 1, after power on plugged in the 4th PSU and verified
            /sys/bus/i2c/drivers/-ibm-cffps/3-006b exist
        3 - Removed 2 PSUs and powered on the BMC with 2 PSUs.
        4 - Use step 3, after power on plugged in the missing PSUs, verified
            bind/Unbind /sys/bus/i2c/drivers/-ibm-cffps/3-006x for appropriate PSU.
        5 - Powered on system with 4 PSUs then removed 2 PSUs. Verified
            unbind done for appropriate PSU.

Change-Id: I325fc0dbb16f9c3b582b739e8440a74e34aae753
Signed-off-by: Faisal Awada <faisal@us.ibm.com>
diff --git a/phosphor-power-supply/power_supply.hpp b/phosphor-power-supply/power_supply.hpp
index da76434..3c6408b 100644
--- a/phosphor-power-supply/power_supply.hpp
+++ b/phosphor-power-supply/power_supply.hpp
@@ -547,6 +547,23 @@
      */
     static double linearToInteger(uint16_t data);
 
+    /**
+     * @brief Retrieve device driver name
+     */
+    const std::string& getDriverName() const
+    {
+        return driverName;
+    }
+
+    /**
+     * @brief Set device driver name
+     * @param[in] newDriver - device driver name.
+     */
+    void setDriverName(const std::string& newDriver)
+    {
+        driverName = newDriver;
+    }
+
   private:
     /**
      * @brief Examine STATUS_WORD for CML (communication, memory, logic fault).
@@ -656,6 +673,10 @@
      * driver for the power supply when it is installed, or unbind the device
      * driver when the power supply is removed.
      *
+     * Note:
+     *    Bind device when device present and i2cbus-i2caddr does not exist
+     *    UnBind device when device not present and i2cbus-i2caddr  exist
+
      * Writes <device> to <path>/bind (or unbind)
      *
      * @param present - when true, will bind the device driver
@@ -784,7 +805,7 @@
     /**
      * @brief The file system path used for binding the device driver.
      */
-    const std::filesystem::path bindPath;
+    std::filesystem::path bindPath;
 
     /**
      * @brief Get the power on status of the psu manager class.